adjective

perfidious

per-FID-ee-us/pərˈfɪdiəs/

Deliberately betraying trust or loyalty.

The perfidious colleague secretly passes the prototype to a rival while shaking the teammate's hand.

A disloyal colleague smiles while shaking a teammate's hand, but secretly passes that teammate's plain prototype model to a rival behind their back.

Etymology

perLatinthrough

+fidLatinfaith, trust

+iousLatincharacterized by

The combination of “through” and “trust” comes from a phrase meaning “to deceive through trustingness.” A perfidious person exploits another's trust, turning what should secure loyalty into an opportunity for betrayal.

In context

Real Crime Storys WW 28:41

Greese's approach was particularly perfidious: she sent women from her work detail behind a pond that served as the camp boundary, whereupon they were shot by guards.

Irma Grese | The Beast of Auschwitz Buchenwald and her heinous atrocities | New documentary 2024

More examples

Her perfidious business partner stole their savings and disappeared.
He calls his friend perfidious for deliberately revealing a secret.
The general's perfidious deal left his own troops defenseless.

Forms

perfidy noun — Deliberate betrayal of trust or loyalty.

perfidiously adverb — In a way that deliberately betrays trust.

Similar words

treacherous: Also describes hidden dangers, not just deliberate betrayal of trust.
disloyal: Broader term for failing to support someone or something.